1. Field of the Invention
The present invention relates to a piezoelectric device. More specifically, the present invention relates to a piezoelectric device utilizing a characteristic of a piezoelectric member as polarized, such as a surface acoustic wave device, a bulk wave piezoelectric resonator device, a piezoelectric transducer, a mechanical resonator device including a piezoelectric transducer and an electroacoustic transducer and the like.
2. Description of the Prior Art
A surface acoustic wave device comprising a surface acoustic wave filter, a surface acoustic wave resonator, a surface acoustic wave delay line and so on is typically composed of a ceramic material having a piezoelectric effect. A typical material comprises ceramics of a barium titanate group, a lead titanate group, a lead titanate zirconate group and the like. Likewise, a bulk wave piezoelectric resonator device is comprised of a ceramic material having a piezoelectric effect. A typical material comprises ceramics of a barium titanate group, a lead titanate group, a lead titanate zirconate group and the like. Of late, a bulk wave piezoelectric resonator device has been used as various kinds of filters, traps, discriminators, oscillators and the like. A piezoelectric device further comprises a piezoelectric transducer, and a mechanical resonator device and an electroacoustic transducer employing such piezoelectric transducer. A piezoelectric transducer also employs, as a typical material, ceramics of a barium titanate group, a lead titanate group, a lead titanate zirconate group and the like. A piezoelectric tuning fork and an electrochemical filter are known as typical examples of a mechanical resonator using a piezoelectric transducer and are used as a selecting device or used as an oscillator in receivers, remote control systems, pocket bells and the like. An electroacoustic transducer employing a piezoelectric ceramic transducer is typically used in a microphone, a pickup, a cartridge, a speaker, a buzzer, a sonar, a ultrasonic cleaner, an ultrasonic machinery, a remote control apparatus and the like, depending upon classifications in terms of applications and the frequency being used ranges from several Hz to several hundreds kHz, with diversification of a vibration wave format from a very low frequency vibration to an audible sound and ultrasonic wave.
A device of high reliability is required in such types of piezoelectric devices and particularly a device of a stabilized characteristic with respect to the environment, particularly an environmental temperature and an elevated temperature.
However, according to a conventional device, a phenomenon often occurred in which a piezoelectric characteristic is degraded when the device is placed in a severe temperature environment and an elevated temperature environment, which was observed by a test called as a special test such as a heat shock test and a high temperature test. Therefore, conventionally various kinds of improvements were proposed; however, a most appropriate approach has not yet been found.
Generally, a ferroelectric plate used in a piezoelectric device must meet the following requirements as important characteristics, i.e. (1) in the case of a filter, for example, an insertion loss, a center frequency and the like will not change depending on the temperature variation, and (2) in the case of a transducer, for example, an electromechanical coupling coefficient is large, and so on. For example, it has been proposed that various kinds of additives are added to a main constituent of ceramics of a lead titanate zirconate group, for example, with an attempt to improve the characteristic by selection of the materials per se. In fact an improvement can be seen to some extent in a heat shock test and a high temperature aging test by thus changing the materials per se, whereby the above described characteristic (1) might be fulfilled; however, conversely a phenomenon was observed in which the above described characteristic (2 ) is degraded.
Furthermore, attempts have been made to improve the above described characteristic (1) by selecting the firing conditions; however, a disadvantage is brought about that the above described characteristic (2) is diversified or degraded, while a control of such firing conditions is difficult and the approach is not suited for production on an industrial basis.